From the European Patent Application No. 50682 (U.S. Pat. No. 4,468,783) an electrode for arc furnaces is known, showing the above-mentioned characteristics. The electrode described in this patent application consists of a metal shaft connected to the active portion by means of a nipple, and has a protective jacket enveloping the metal shaft.
A combination electrode is disclosed in the U.S. Pat. No. 4,121,042, the water-cooled metal shaft of which being directly exposed, i.e. without any protective jacket, to the atmosphere of the furnace and to the attack of the arc. The electrode has to be clamped directly on the metal shaft.
The UK Patent Application No. 2,037,549 shows a combination electrode having a metal shaft which consists of two water pipes surrounded by a jacket of steel produced from three coaxial cylinders, thus forming two annular spaces forming part of a water cooling circuit. The described jacket surrounds the inner pipes substantially over their entire length, and the electrode is clamped by this jacket.
The electrode according to the European Application No. 50682 has very good operating characteristics and is already in use in a number of arc furnaces. Since the principal motive promoting today the development of water-cooled electrodes is to reduce the cost of steel production, new possibilities are desired to save additional cost. For this reason, further efforts have been made in order to reduce the maintenance cost of the electrode known from the European Application No. 50682, without any risk for the good operating characteristics.
The electrode according to U.S. Pat. No. 4,121,042 has the drawback that its metal part consists of a single integral element so that, with the peripheral metal wall damaged by an arc or by a mechanic impact of heavy scrap pieces, the whole electrode has to be disassembled and reassembled after exchange of the damaged parts.
The electrode described in British Patent No. 2,037,549 has a steel jacket also connected to the inner cooling pipes of the electrode, and therefore cannot be exchanged.
Furthermore, it is disadvantageous that clamping of the electrode is effected by its jacket, since the latter has to be for this purpose of an enormous mechanical resistance in order to absorb the clamping forces being in the order of 20 to 40 tons.
The U.S. Pat. No. 4,291,190 discloses an electrode holder according to the one mentioned in the preamble which comprises several vertically assembled water pipes for cooling a nipple at the lower end of the electrode holder. The water pipes are embedded within a cylinder of refractory material which in turn is encompassed by a hollow cylinder of steel which snugly fits to the refractory cylinder. It is claimed in this patent, that the metal cylinder is electrically insulated from the current conducting water pipes by the refractory cylinder. However, the same refractory cylinder is supposed to sufficiently cool the metal cylinder by heat conduction to the water pipes.
It is well known, that good heat conductors are good electrical conductors too and vice versa. The above electrode has therefore the disadvantage, if the metal cylinder should be sufficiently cooled, that the electrical insulation of the metal cylinder from the water pipes is not perfect, an effect which becomes still stronger with higher temperatures. It is, however of great importance to keep the metal sheath electrically insulated from all parts which are kept on electrode potential, since a non-insulated metal sheath would be exposed to the formation of electric arcs between the sheath and scrap which comes close to it, and would therefore quickly be damaged.